Space-based surveillance systems use infrared detectors coupled to computerized data processors for monitoring heated objects and their movements in the atmosphere below and on the ground. The function of infrared detectors is to respond to energy of a wave length within some particular portion of the infrared region. Heated objects dissipate thermal energy having characteristic wave lengths within the infrared spectrum. The infrared spectrum covers a wide range of wave lengths, from about 0.75 micrometers to 1 millimeter. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. Detectors are selected in accordance with their sensitivity in the range of interests to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detectors is also selected in view of the intended detection functions.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements the outputs of which are connected through sophisticated processing circuitry. It is difficult, however, to actually construct structures that make a million or more detector elements and associated circuitry in a reliable and practical manner. Practical applications for contemporary infrared detection systems necessitate that further advances be made in the reliability and economical production of assemblies of detector arrays and accompanying circuitry.
Because the array material is very thin, less than 0.005 inches, difficulties arise in attaching the array material to the base of the module. One such difficulty is the inability of the detector material to absorb forces generated by the mismatched coefficient of expansion between the module and the array material. An additional difficulty encountered is providing a means for testing the reliability of the individual detector elements. Where the detector material is applied directly to the module body it is difficult to isolate a fault that may be attributable to either the detector elements, module wiring or processing elements. Schmitz, U.S. Pat. No. 4,792,672, also assigned to Grumman Aerospace Corporation, addressed those issues with a buffer board disposed intermediate the detector array segment and the multi-layer module. The buffer board facilitates assembly of the detectors to the multi-layer module, and also enhances the structural characteristics and separate testability of the system components.
Considerable difficulties are also presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment. Indium bumps and flip-chip bump bonding techniques are commonly used for high-density interconnection in integrated circuitry, such as here between infrared detector rays and signal processing modules. Indium bumps 30 to 40 microns in diameter and spaced approximately 100 microns apart (center to center) are typically formed in arrays upon two substrate surfaces to be electrically connected, such that the indium bumps will fuse when brought into contact and forced together. Each indium bump may be connected to a conductive conduit which provides electrical communication to integrated circuitry formed upon the substrate. In the present application the opposing substrate supports an infrared detector array containing 1000 or more detector pixels are formed upon a semiconductor substrate, and must be electrically connected to signal conditioning electronics formed upon the other semiconductor substrate.
The conventional positioning of detector arrays relative to multi-layer modules for attachment and electrical connection is a difficult process. Careful positioning and a means to maintain the position under various temperature conditions is required. Small sections of detectors are positioned and held by precision tooling, until a bonding media such as epoxy can cure to form a permanent structure. Epoxy materials may shrink, or absorb moisture and expand, changing the position of the detectors.
Although prior art practices of forming indium contact bumps and bonding arrays of infrared detectors to signal processing modules have proven generally suitable for their intended purposes, they possess inherent deficiencies which detract from their overall effectiveness in the marketplace. In view of the shortcomings of the prior art, it is desirable to provide a method for bonding integrated circuit modules together with bump contacts which is simpler to practice and which has a higher yield than contemporary processes. It is also desirable to circumvent the use of adhesives during the positioning of the detector arrays, and instead rely on another method of manufacture.